The need for miniaturization of optical part packages and higher functionality has increased year after year, and integration of optical devices has proceeded with this. For this reason, it is required to consider methods of integrating the optical devices, material for selection thereof, and device structures.
First, when different functional elements such as a wavelength variable filter or a modulator are integrated into a light-emitting element such as a laser, it is preferable that the materials of each of the functional elements are formed using an optimal one in accordance with each of the characteristics. Materials used in the optical devices include a compound semiconductor-based material, a silicon-based material, and a quartz-based material.
Next, in the functional elements in which a compound semiconductor is used for a portion of a region or the entire region among the materials, it is preferable that from the viewpoints of the device size, high-speed responsiveness, reliability and the like, structures of each of the functional elements are formed using optimal one in accordance with each of the characteristics. The structures include a core structure and a waveguide structure.
First, the core structure will be described. The core structure can be divided into an active region with a layer having a small band-gap energy in order to obtain an optical gain in the objective wavelength, and a passive region with a layer having a large band-gap energy. The band gap can be adjusted by the layer thickness, the crystal composition ratio, and crystal distortion caused by the crystal composition ratio. In addition, the active region and the passive region can, respectively, have different physical properties by changing the types of constituent atom materials.
Next, the waveguide structure will be described. The three main types of waveguide structures often used in the optical device are a deep-ridge waveguide structure, a rib waveguide structure, and an embedded waveguide structure (FIG. 7).
FIG. 7A is a schematic diagram illustrating an example of a deep-ridge waveguide structure. As shown in FIG. 7A, a ridge-shaped lamination composed of clads 11 and a core 12 interposed between the clads 11 is formed on a substrate 10. A deep-ridge waveguide structure has a shape in which the portion on the side of the waveguide is replaced by a low refractive index material such as air or a dielectric across the core structure in the vertically direction. For this reason, the structure is characterized in that a refractive index contrast between a semiconductor and a low refractive index material is large, and that radiation loss hardly increases even when the waveguide is steeply bent. Consequently, the deep-ridge waveguide structure is used in forming a device, in which a curved waveguide is heavily used, in a small size. In addition, it is possible to reduce the electrical capacitance by filling both of the sides of the deep-ridge waveguide with a low refractive index, that is, a low refractive index material. In this case, since the charging time of the capacitance portion at the time of signal modulation is shortened, the structure is also used in a device requiring high-speed operation.
FIG. 7B is a schematic diagram illustrating an example of a rib waveguide structure. As shown in FIG. 7B, a lamination composed of the clads 11 and the core 12 interposed between the clads 11 is formed on the substrate 10, and the clad 11 formed on the upper portion of the core 12 is formed in a ridge shape. The rib waveguide structure has a shape in which a clad layer above the core structure is replaced by a low refractive index material such as air or a dielectric. In this structure, since the core structure is not processed in order to form the waveguide, lattice defects hardly occur. As a result, even when a localized rise in temperature due to current injection occurs, an increase in lattice defects leading to the device degradation is also hardly brought about, and thus good reliability is obtained. Consequently, the rib waveguide structure is often used in gain devices associated with the current injection, such as a laser, an LED, and an optical amplifier.
FIG. 7C is a schematic diagram illustrating an example of an embedded waveguide structure. As shown in FIG. 7C, the clad 11 and the core 12 embedded in the clad 11 are formed on the substrate 10. The embedded waveguide structure has a shape in which the portion on the side of the waveguide is replaced by a semiconductor material having a smaller refractive index than that of the core structure. In this structure, since damage caused by processing for waveguide formation is restored by crystal regrowth, lattice defects hardly occur. As a result, similarly to the rib waveguide, even when a localized rise in temperature due to current injection occurs, an increase in lattice defects leading to the device degradation is also hardly brought about, and thus good reliability is obtained. Consequently, the embedded waveguide structure is often used in gain devices associated with the current injection, such as a laser, an LED, and an optical amplifier. In addition, the embedded waveguide structure has a defect that a process is complicated, but has better carrier confinement than that of the rib waveguide structure. In addition, like the deep-ridge waveguide structure, a leakage current through the lateral side of an exposed semiconductor is also hardly generated in the embedded waveguide structure. For this reason, when the structure is applied to a laser, it is possible to reduce a threshold, and to perform a high-efficiency operation.
There is no waveguide structure or core structure which is the most suitable to any and all devices. Therefore, it is important to use the waveguide structure or the core structure suitable to each of the devices in order to improve performance as the integrated device as a whole.
One of monolithic integration methods of two types of core structures is a known butt-joint technique in which a portion of the core structure is processed by dry etching and the like, and a separate core structure is newly formed by crystal growth such as Metal-Organic Vapor Phase Epitaxy (MOVPE) or Chemical Beam Epitaxy (CBE) (Patent Document 1).
Since each of the waveguide structures has a significantly different cross-sectional shape, the electric field distributions in a waveguide mode are also different from each other. When a direct connection is performed, a portion of a non-conforming component of the electric field distribution in the waveguide mode becomes reflected return light. The reflected return light causes laser noise. Thus, even when the relative amount of return light is an extremely small amount of 10−6 or so, a change in the characteristics occurs. Specifically, the optical output, the number of oscillation modes, the oscillation spectrum, the noise intensity, and the response output waveform at the time of modulation are changed. An associated technique includes, for example, Non-Patent Document 1. In this method, the waveguide width is adjusted so as to match the electric field distributions in the waveguide mode to each other in the interface between the deep-ridge structure and the embedded structure, and the shape of the waveguide in the vicinity of the interface is gradually changed to a tapered shape. Thereby, a sudden change in the waveguide mode is suppressed, and the reflected return light is suppressed.